1. Technical Field
This invention relates to apparatus for locating the position of a momentary, localized force applied to the surface of a visual display and, more particularly, to an improvement to the alternating current coupling and computer processing circuitry employed in such apparatus.
2. Description of the Prior Art
From the earliest days of interactive computer use, a need has been felt to visually communicate with a computer in a direct, efficient manner. In the desired mode of operation, a computer terminal having a visual display presents to the user a plurality of alternative choices on the display. The user selects one desired alternative by simply touching or tapping the display at a location corresponding to the desired alternative. The computer terminal then reacts to the momentary, localized force of the touch by identifying that the touch has occurred at the location of the selected alternative presented on the display. The terminal may then perform according to the choice made by its user. While the basic goal is simply stated, it will be seen that a practical solution is dependent on the analysis of several problems inherent in prior art position locating systems.
Peronneau et al, U.S. Pat. No. 3,657,475, discloses a position indicating system comprising a rigid plate, a plurality of force sensing devices, and a processing unit. When a localized force is applied to an exposed surface of the plate, the force sensing devices provide the processing unit with electrical signals which are proportional to the localized force. The processing system calculates the coordinates of the location of the localized force touching the plate. Equations from the laws of statics are employed; namely, that the sum of the moments about an axis equals 0.
Peronneau provides an integrator for collecting force sensing device data in a first embodiment of his invention. In this embodiment, both the static and low frequency dynamic characteristics of the touch are provided to the processing unit for calculating the coordinates of the localized force. However, the static forces due to the weight of the plate are not separated from the dynamic characteristics of the applied touch, and this presents an admitted problem to Peronneau. He discloses that in a second, preferred embodiment of his invention the dynamic characteristics of the touch are analyzed. However, Peronneau does not clearly disclose how the dynamic characteristics of the touch are separated from the static characteristics.
Roeber, U.S. Pat. No. 4,121,049, discloses a position and force measurement system comprising touch plate apparatus and a digital processing system. The touch plate apparatus comprises two similar rectangular rigid surfaces one of which being exposed to the application of a touch force. The edges of the surfaces are joined by semicircular springs upon which are mounted strain gauge sensors. Stresses detected by the sensors are proportional to the magnitude of the downward movement of the exposed surface when a force is applied to it. Accordingly, the magnitude of the tangential force and the coordinates of its location may be calculated.
An improvement over Peronneau which Roeber discloses is a threshold comparator amplifier for insuring that the applied touch is above a predetermined threshold level. A shortcoming of the use of the comparator amplifier is that the predetermined threshold level does not automatically adapt to the environment or the quiescent state orientation of the touch plate apparatus. For example, if the Roeber touch plate apparatus is rotated from a vertical to a more horizontal orientation, the weight of the touch plate causes a corresponding increase in the stress detected by the strain gauge sensors. As a result, the threshold level must be manually increased to compensate for the increased stress.
Roeber also discloses an improved digital processing system for calculating the location coordinates. Roeber discloses that the digital processing system comprises a clocking and sample request circuit and an analog to digital converter. Responsive to the clocking circuit, the multiplexer and analog to digital converter provide the digital processor with digitized samples of the static characteristics of the touch. No sampling rate is disclosed; however, it is disclosed that the digital processor repeatedly computes coordinate values for each set of samples until the magnitude of the force of the touch falls below the predetermined threshold sensing level of the threshold comparator circuit.
Other touch sensitive systems have been developed which do not react to the force of a touch. These systems employ a great variety of technologies including ultrasonic echo timers, resistive film contact sensors, optical beam detectors including infrared beam detectors and light pens. These technologies suffer, however, from high costs. On the other hand, their commercial success demonstrates the great demand for a touch sensitive human interface with a visual display.
Systems for selection of menus and items of menus have been developed outside the art of touch sensitive displays. These include joy stick and cursor systems and other keyboard systems. The several alternatives suggest means for positioning a cursor on a screen to choose an appropriate menu or an item from a menu. In general, they provide external means for causing the cursor to move about the screen to the desired location. It is believed, however, that it is more desirable to directly effectuate these functions without using external apparatus.
Accordingly, it is believed that there remains a requirement for a dynamic position locating system for directly and efficiently calculating the coordinates of a momentary touch so that a computer processing system may identify a selected alternative and perform according to the choice made by its user.